REVIEW Thyroid hormone and central nervous system development

It is well established that the thyroid status of neonates and children has a significant long-term impact on their behaviour, locomotor ability, speech, hearing and cognition (Legrand 1986). Delay in restoring normal thyroid status in the neonate can lead to irreversible damage. Prompt thyroid supplementation following the diagnosis of neonatal hypothyroidism can restore neurodevelopment to within the normal range (Fisher 1979). Even so, there are still subtle abnormalities of language, visuo-spatial impairments and lower mean IQs later in childhood compared with euthyroid controls. This implies that brain development may, in part, be thyroid hormone sensitive not only in the neonatal period but also prior to birth (New England Congenital Collaborative 1990, Heyerdahl 1991). Development of different areas of central nervous sytem has been associated with the timing and duration of thyroid hormone deficiency, suggesting that there are critical periods during which various parts of the brain are sensitive to thyroid hormone supply (Rovet et al. 1992).